# merge_sort(nums,0,3)
#     mid = 1
#     merge_sort(nums, 0, 1)
#     merge_sort(nums, 2, 3)
#     merge(nums, 0, 1, 3) 第三次执行merge


# merge_sort(nums, 0, 1)
#     mid = 0
#     merge_sort(nums, 0, 0)
#     merge_sort(nums, 1, 1)
#     merge(nums, 0, 0, 1)  第一次执行merge

# merge_sort(nums, 2, 3)
#     mid = 2
#     merge_sort(nums, 2, 2)
#     merge_sort(nums, 3, 3)
#     merge(nums, 2, 2, 3) 第二次执行merge

# 结论：
#     先 merge(nums, 0, 0, 1)
#     再 merge(nums, 2, 2, 3)
#     最后 merge(nums, 0, 1, 3)